基于一种新型扫描模式的不完全数据CT重建

扫描模式是直接决定图像重建实现方案的重要因素之一.随着CT技术的不断发展,扫描模式也在不断地更新换代.以降低剂量、工程条件受限为应用背景的不完全数据图像重建和扫描模式有着更加密切的联系.立足于不完全数据的图像重建,定制了一种直线轨迹的扫描模式,并在这种扫描模式下,探讨半区域、外部区域、内部区域的不完全数据的精确重建.数...     

工业CT教学实验仪图像重建的设计

工业CT教学实验仪被应用于物理和放射医学专业的教学,图像重建是教学实验仪软件的重要组成部分.论文阐述了CT的物理原理和第一代CT重建算法,描述了工业CT教学实验仪的扫描过程;分析了旋转中心偏移的伪影,实现了中心偏移校正方法,设计了图像重建软件,实际应用表明此软件重建图像清晰质量较高.     

基于硬件实现的锥束CT图像重建系统的存储机制设计

为了实现三维锥束CT图像重建加速系统的小型化,建立了基于FPGA的三维图像重建系统。并对该系统中所采用的FDK重建算法所需的数据存储量和数据传输量以及由SDRAM、SRAM和FPGA内部缓存组成的存储系统的数据吞吐率进行了研究。首先,根据FDK算法的滤波与反投影两个步骤介绍了三维锥束CT图像重建系统的数据规模。接着,介绍了一种以SDRAM为外部主存,以SRAM为外部缓存和以FPGA内部SRAM资源作为内部高速缓存的存储机制。然后,介绍了该存储机制的实现方法以及测试方法。最后对该三维图像重建系统的数据吞吐能力进行了测试,并将之与FDK算法所需的数据传输量进行了对比分析。试验结果表明:该存储机制的数据连续存取速度为151.9MB/s,数据随机存取速度为100MB/s,基本满足小规模的三维图像重建的数据存储与传输带宽的要求。     

大型集装箱CT系统图像重建研究

研究了大型集装箱CT系统的图像重建技术,给出了卷积反投影算法的基本原理、实现过程,以及目前得到的重建图像。经实际检验,应用该算法重建的图像清晰,能够识别铸造工件中的孔及树木的年轮。     

数字图象重建的一种综合快速算法

本文提出一种综合快速算法,可直接快速地计算重建图象平面上任何一点的检测角,并采用三次B样条函数对滤波的投影数据进行内插,来代替反投影时的内插。实验表明,在保证同样的主客观保真度下,此算法的内插计算量约为原来算法的1/12;当象素数N增大时,计算量的减少更为显著。对端点条件的影响进行了分析,证明用此法插值所造成的误差以相当高的速率由边缘向中心衰减,大约每向中心移动两个象素其误差减少一个数量级。     

A New Approach to Direct Current Integration

A new all-solid state direct-current integrator integrates currents as low as 10-11 ampere with an accuracy of 1%. The operating principle is similar to that of more conventional current integrators in that the voltage across a storage capacitor in the input circuit is monitored by an electrometer type operational amplifier, the output of which triggers a discriminator at a given voltage level. The discriminator generates a reset pulse which removes a fixed charge from the integrating capacitor. This is where the similarity ends. The diode pump circuit, which has long been recognized as an ideal circuit for removing precise quantities of charge from the integrating capacitor, has been replaced by complementary silicon planar transistors that operate in the inverted-mode and serve as current switches. These transistors normally have both their base-emitter and base-collector junctions reverse biased and look like open switches with leakage currents of about 10-13 ampere. Reset pulses switch these transistors from the reverse-biased state to the active state for a precise time during which a constant current flows to or from the integrating capacitor. Therefore, each reset pulse removes from the integrating capacitor a precise charge that is the product of the current and the period of the active state. By the employment of sufficient gain in the operational amplifier to maintain small voltage excursions at the input, the total effective leakage may be kept as low as 10-14 ampere.     

SPECT准直器几何及图像重建算法的进展

本文简要介绍了近年来ＳＰＥＣＴ准直器几何设计和对应图像重建计算方法的最新进展以及当前图像重建研究的一些热点。     

A New Approach to the Design of Amplitude to Code Converters

A pulse amplitude to code converter (ACC) for use in multichannel analysis with a computer based data aquisition systems is developed. A new design concept was choosen based on pulse sampling and storage. The approach has minimum differential and integral non-linearity, maximum stability and modular structure. The pulse storage is done on a switched condenser and the amplitude to code conversion carried out using the up-down integration method. Encapsulated and integrated modules, as well as field effect switches are used in the realization. The resulting ACC is simple, accurate, stable and requires a little development effort. An experimental unit with 1024 channel resolution was built and interfaced to a mini-computer. Tests showed very promising results.     

基于Radon变换导数的精确三维CT图像重建理论与改进方法介绍

本文主要介绍了一种基于Ｒａｄｏｎ变换导数的精确３Ｄ－ＣＴ图像重建的改进方法，它的计算复杂性和成像质量与传统方法相比有很大改善。     

A New Approach to High Resolution Timing with Photomultipliers

A new technique (Double Sampling) of photomultiplier pulse processing is studied in order to improve the time resolution. Two time references, t1, and t2, are obtained from two different processing systems, into which the fast pulse of the photomultiplier is fed. If the condition: (t1 - t2) -?t < (t1 -t2) < (t1 - t2) + ?t is satisfied, the pulse is accepted for the time analysis. ?t is a predetermined parameter of the proposed system. A theoretical study has been carried out in order to compare this processing method with the ones already used.     

用于核武器部件检查的新型X射线三维成像系统

对美国劳伦斯.利弗莫尔国家实验室研制的新型X射线层析成像系统(密闭大型光学闪烁体屏和成像系统能够用来对核武器关键部件进行高分辨率三维成像)的研发背景、系统概况、工作原理和关键技术进行了介绍,并对该系统未来的应用情况和发展前景进行了展望。     

旋转镜像NLM约束稀疏角度CT重建

提出了基于旋转镜像变换的自适应非局部平均算法,用于稀疏角度CT重建。设计基于相似窗旋转镜像变换的相似性测度以避免图像边缘模糊;并根据像素梯度及重建迭代次数自适应调整滤波参数,有效保护图像结构信息。新算法主要包含三个步骤:代数重建、非负修正及RAMNLM滤波,循环执行以上步骤直至满足迭代收敛准则。以Shepp-Logan体模进行仿真重建结果表明,新算法可有效抑制噪声并恢复图像细节,显著提高图像质量。     

一种集装箱检查系统辐射图像的伪彩色变换方法

为提高钴-60集装箱检查系统所得辐射图像的可识别度,提出一种基于图像灰度特性的伪彩色变换方法。该方法首先对辐射图像进行预处理,保证本底灰度的像素映射到灰度0附近,然后根据辐射图像的直方图,动态地将颜色表中的颜色分配到不同灰度上,并根据奥斯特瓦尔德颜色体系设置颜色表,使得彩色的显示更有层次感。该方法应用于某海关集装箱检查系统,取得了较好的显示效果。     

A New Three-Dimensional Code for Simulation of Ion Beam Extraction： Ion Optics Simulator

A new thee-dimensional code, ion optics simulator （IOS）, to simulate ion beam extraction is developed in visual C＋＋ language. The theoretical model, the flowchart of code, and the results of calculation as an example are presented.     

Design of a New Faraday Cup to Measure the Beam Current of an Ion Source with Residual Gas

Different parameters affecting the faraday cup (FC) performance are investigated. It is shown that the FC design depends on the incident particle beam parameters. If FC is used to measure the beam current of a dense plasma focus device or if FC is used to measure the beam current of an ion source where there is residual gas, it should allow the secondary electrons to escape from the cup. Then the effect of the escaped electrons must be estimated somehow. Otherwise, to ease the calculations, FC should be designed to recapture the secondary electrons. Here, we propose a novel FC to measure the particle beam current of an ion source where residual gas or electromagnetic noise exists. To obtain precise beam current measurement, FC performance is simulated by CST Studio soft ware. In addition, a method is proposed for the analysis of the FC signals.     

A Microchannel Plate Based Detector for a Heavy Ion Beam Spectrometer

The design parameters and operating characteristics of the detector used in the Brutus and Fannie heavy ion beam spectrometers at the SuperHILAC facility are described. The detector utilizes a 25 mm diameter microchannel plate array to obtain gains of 102 to 108 with a linear dynamic range of 103. It has had over three years of almost maintenance-free service, detecting ion beams from carbon to xenon with energies between 1.2 and 8.5 MeV per nucleon.     

New Approach to the Nuclear Fuel Reprocessing in Non-acidic Aqueous Solutions

A new nuclear fuel reprocessing method based on the anodic dissolution of spent fuels in aqueous alkaline solutions (Na₂CO₃-NaHCO₃) has been proposed. Experiments of the anodic dissolution were performed by using a simulated spent fuel in a Na₂CO₃-NaHCO₃ solution. Uranyl ions produced anodically were present in the solution as stable carbonato complexes, and at the same time, most of the simulated fission products (FP) were precipitated as hydroxo or carbonate compounds. Under this condition, Cs of an alkali metal group was dissolved in the solution and precipitated by adding sodium tetraphenylborate. Uranyl ion was recovered as hydroxo compounds by adding NaOH to the solution after removing precipitates of the simulated FP. In view of waste disposal, ⁹⁹Tc having a long half-life should be removed. Precipitation behavior of Tc(VII) was examined by using Re(VII) as a simulant of Tc(VII). It was found that Re(VII) species are completely removed as a precipitate by adding tetraphenylphosphonium chloride. A large amount of Na used in the present method was recovered as NaHCO₃ by blowing CO₂ into alkaline solutions. As a result, it was clarified that the proposed method is fundamentally possible as a new reprocessing method.